Interpretive Summary: The role of rangelands in the regulation of atmospheric carbon dioxide concentrations is a critical issue in global climate change research. Rangelands are complex ecosystems that occupy more than 40% of the land area in the world and USA. We studied the effects of short-duration intensive grazing on carbon dioxide flux on small plots located on a silty range site in the northern mixed-grass prairie of Montana. Treatments were no grazing, or grazing during mid-May or mid-July in 1996, 1997, and 1998. Date were collected from mid-April to mid-October at about 30 day intervals to determine effects of grazing on aboveground and belowground plant biomass, carbon dioxide movement, and soil organic C. Uptake of carbon dioxide was greatest during spring and early summer, peak periods of precipitation and green biomass. Grazing removed an average of 68 to 78% of the green standing crop with a subsequent reduction in carbon dioxide uptake of 175% in May and 109% in July. Grazing in May reduced carbon dioxide uptake for 30 days in 2 of the 3 years, whereas, grazing in July reduced carbon dioxide flux only in 1998. Residual effects of grazing, however, declined in late summer and autumn with the onset of plant maturation. The potential C sink in the mixed-grass prairie of the Northern Great Plains appears to be small, and will vary through time with intensity and timing of grazing as it interacts with climatic conditions.

Technical Abstract:
The role of rangelands in the regulation of atmospheric CO2 concentrations is a critical issue in global climate change research. Rangelands are complex ecosystems that occupy more than 40% of the land area in the world and USA. We studied the effects of seasonal grazing on CO2 flux on small plots located on a silty range site in the northern mixed-grass prairie with an Eapa fine loam soil. Treatments were no grazing, or short-duration intensive grazing during mid-May or mid-July in 1996, 1997, and 1998. Date were collected from mid-April to mid-October at about 30 day intervals to estimate standing crop, leaf area, soil organic C, root mass to a 30-cm soil depth, and diurnal variation of CO2 flux and soil respiration (at 0800, 1200, 1600, and 2400 hours) in closed chambers. Uptake of CO2 was greatest during spring and early summer, peak periods of precipitation and green biomass. Grazing removed an average of 68 to 78% of the green standing crop with a subsequent reduction in CO2 uptake of 175% in May and 109% in July. Grazing in May reduced CO2 uptake for 30 days in 2 of the 3 years, whereas, grazing in July reduced CO2 flux only in 1998. Residual effects of grazing, however, declined in late summer and autumn with the onset of plant maturation. The potential C sink in the mixed-grass prairie of the Northern Great Plains appears to be small, and will vary through time with intensity and timing of grazing as it interacts with climatic conditions.